Air proportional counter



-2 Sheets-Sheet l1 INVENTOR. .l/1v1 ;Zapasazar /w a, M'

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J A SIMPSON, JR

AIR PROPORTIONAL COUNTER Dec. 5, 1950 Filed Deo. 31, 1946 Dec. 5, 1950J. A. SIMPSON, JR 2.532,955l

AIR PRoPoRTIoNAL COUNTER Filed Dec. :51, 194e 2 sheets-sheet 2 f8 I @4Zff .f y... y A f77-HHH v 5228 Wma @WJ an improved center-wire suspensionfor Lproportional counters.

It is a further object of the invention to provide an air proportionalcounter which is not subject to spurious counts and discharges.

It is a still further object to provide an electrically conductingwindow in the cathode of a proportional counter. y

It is a still further object of the invention to provide an improvedproportional counter having parallel plate cathodes and a lplurality ofequally spaced anode center-wires.

The manner in which the foregoing and other objects of the invention areaccomplished is i1- lustrated inthe embodiment of the attached drawing,in which:

Fig. 1 is a plan view of a proportional counter with covering screens tobe described below, partially broken away to show the internalconstruction.

Fig. 2 is a longitudinal cross sectional view taken along the line 2-2of Fig. 1 in the direction indicated by arrows;

Fig. 3 is a transverse cross sectional view of the proportional countertaken along the line 3--3 of Fig. 2 inthe direction indicated by thearrows;-

Fig. 4 is an enlarged fragmentary cross sectional view of a portion ofthe counter illustrated in Fig. l, showing certain details of themounting of the center wires thereof; V

Fig. 5 is a fragmentary cross sectional view illustrating the mountingof an insulator shown in Fig. 4 taken along the line 5-5 of Fig. 4 inthe direction illustrated by arrows; and

Fig. 6 is an enlarged perspective View illustrating the manner ofsuspension of the centerwires of the counter of Fig. 1.

Referring rst to Figs. 1 and 2, a rectangular back plate lll has mountedthereon as by screws at the outer perimeter thereof two Vside walls I2and two end walls I4, for example of aluminum, defining the body of theproportional counter. The interior dimensions may be, say, 5 inches byl2 inches by inch. On the interior surface of each of the side walls I2are two grooves I6 adapted to receive insulators I8. As appears moreclearly in the transverse View of Fig. 3, each of the insulators I8consists of a substantially rectangular bar of an insulating material,preferably polystyrene, extending across the counter from one side wallI2 to the other. Spaced along the edge of each of the insulators I8 aretransverse notches 20 extending part way through the insulator. Asillustrated in Fig. 3, the insulators I8 are wider at the extreme endsthereof than at the central portion in which the notches 20 appear, thusleaving air gaps 22 between the insulator I8 and the back plate Ill andbetween the insulator I8 and the upper conducting surface hereinafter tobe described.

- Centrally of the back plate ID adjacent one end thereof is an aperture24 (Figs. 1 and 2) through which'enters the center conductor 26 of aconventional coaxial cable connector 21 which is fastened to the backplate II) by conventional means. Two insulating posts 23, preferably ofpolystyrene,'are equally spaced from the coaxial cable collector 21along a transverse line slightly forward of the connector 21. Theinsulating posts 28, as appears more clearly in Fig. 5, are fastened tothe back plate I0 by screws 30 extending therethrough. the posts 28being internally threaded for this purpose. The

edge thereof.

therein adapted to receive in tight-fitting fashion a rod 32, forexample of brass. The rod 32, for example 1/8 inch in diameter, issupported transversely of the counter by the insulating posts 28. Aconnecting wire 34 is soldered at one end to the center conductor 26 ofthe connector 2 and at the other end to the central portion of the rod32. Extending transversely of the rod 32, and thus longitudinally Vofthe counter, are small apertures 3e in the rod 32, as clearly appears inFig. 4.

Threaded through each of the apertures 3% is the end of a coiled spring38. TheV end of each of the coiled springs 38 is prevented from comingout of the aperture 36 by a right-angle bend at the end of the spring.The other ends of the springs 38, as more clearly shown in Flg. 6, arebent into the lorm of small hooks fill which support the center-wires 42of the counter. The center-wires 42 are preferably of'l mil tungstenwire. At each end of the wire i2 is a small conducting ball 44,preferably spherical, for example of silver solder, substantiallyintegral with the wire 42. rlhe ball 44' at one end of the wire 42 isretained in place by the hook 40 which is smaller in diameter than theball 44 but greater in diameter than the wire 42. The spring t exertstension on the wire 42. Each wire l2 traverses the corresponding notch20 in the nrst insulator I3 and extends to the other end of the counterwhere the other end of the wire l2 is held in place by the otherspherical ball lili which is larger in diameter than the width of thenotch 2u in the second insulator I3, so that the tension in the spring38 holds the ball tightly against the notch 2li, and thus secures thecenter-wire in place. It will be seen that the center-wires 42 may bereadily replaced without the necessity of soldering.

Over the lace or the counter, supported by the walls I2 and I of thecounter is a rectangular frame iii constituting the support for a window48, which is secured, as by cement, to the The window 48 may be of nylonof less than 0.5 mil thicliness, and preferably of 0.2 mil thickness.The window 48 extends ovel` approximately all the face of the countercorresponding to the region between insulators I8 near either endthereof. The window 48 has deposlted upon the inner surface thereof athin coating of a conducting substance such as graphite (not visible inthe drawing). superimposed upon the frame 46 for the window 48 is anaddltional frame 5t, preferably of a plastic such as polymerized methylmethacrylate, commonly called lucite. The frame 5f) constitutes thesupport for a wire mesh screen 52 preferably of stainless steel whichextends thereacross and the edges of which are preferably molded intothe frame 5t. The frames 4b and 56 are secured to the walls l2 and lllby screws 4l and 5I, respectively. The window 4S constitutes, with theback plate I0, a parallel-plate cathode, the anode center wires 2preferably being approximately midway therebetween. The window 43permits alpha particles from the exterior to enter the counter. Thescreen 52 protects the window 48 against both protrusions and dirtaccumulations on surfaces, and may readily be removed for cleaning, forexample in an acid bath if made of acid-resistant materials such asuggested above.

In the construction of the counter illustrated in the drawing the jointsbetween the Various' parts defining the counter body are preferablyair-permeable for it'wiu te readily seen that otherwise apressuredifferential could be created acossfthe delicate window 48. t `is notneces-A sary, hwever, to provide any merture to yallow thefinlux of airSince in the absenceof affirmative sealing means such as greases,therewill be siicient leakage `between the interior` of the counter and theambient atmosphere to insure that no substantial pressure diiTerentialbetween theinside and the outside of the counter may exist. This will betrue even with the greatest care in `machining and lassembling of theconn-l ter. `Since it isnecessary for proper operation thatthe counterbe absolutely free of dust, it is highly desirable that the partsdescribed abve, particularly those between which air leakage will occur,be'machined to the finest possible tolerances soas to allow a minimum ofdust to enter the counter. l

The structure ofthe embodiment of the presi entinvention illustrated inthe drawing having thus 'been described, the theory of operation and thebasic principles of the design may now be disciissed.

Air is admittedly not an optimum choice from the theoretical standpointof a filling for aproportional counter. However, the use of air atatmospheric pressure as such ionization medium, as illustrated in theexemplary enibodilnet of the drawing, makes possible the performance ofcounting operations which heretofore could not adequately be performed.For example, a `prportional counter such as that illustrated in thedrawing may be used in making routine surveys of areas suspected ofcontamination by alpha particle emitting substances. l

The general theory of proportional counters is wiell kncwn in the artand will vnot be set forth here. The theory and observations below will,therefore, be confined principally to phenomena in respect to which airsubstantially differs 'from other ionizing media. y y

The oxygen and carbon dioxide content of air introduces the problem ofnegative ion formation. "The electron attachment for oxygen is' 1.73jelectronvolts. This results in serious lss oielectrons formed in Vtheprimary ionization of air by an alpha particle since some oi 'the elec;trons Yprimarily induced will becerne attached to oxygen atoinsbeforecausing additional ionization. This is especially true in theouter, or low intensity, field region of the proportional counter. Suchan eiect is observed ras an apparent decrease in counter efliciency. Athigh voltages,

furthermore, this results in spurious counts since' the electron whichhas become attached tothe oxygen atom may again become detached longafter the original ionizing event, and thus cause a spurious count.Although these eifects are irnportant in using such a counter as hereinde#I scribed for particle energy measurements, they are practicallynegligible for determining the nurnberof ionizing events, as theoperating Volt-4 age of the counter is selected so as to be considerably below the semi-proportional region.

"Thelpresenee of nitrogen in the counter re'- sults a relatively highoperating potential. This requires that very small center-wire diar`net'er's be used in order to obtain adequate gaslainpliication atreasonably moderate potentials. The center-'wires should therefore beless than 3 milsin diameterand preferably l ALrriil or less.

In order to design air proportional conters, it 'isfnecessary tounderstand the principal diffeifefces between theele'ctrical' dischargeniecliaera few of the el l In addition, any photons the gas.` Underthese ni'sil iii "a "eas the *disch mechanism in aumpixture f Theproperties" of the passe the" "h, po are f Qi" f portional region f lplied betweii'the required to give' .angels ron enough produceiiiiatio'ii last before being 0j A a at nie in ythe potencial differenceionization and 'mitiiineatid Many of theieiecuis .1p y Charge becomeexert ind,

erranti". fi he effect .of the .d l stitutes one ofthe 'pr' theVelischalr ge reces` thatlnre ro the polyatoginic' .gas-are` Messa" ilybecanse of the wide photojfabso" ti such typeso-f nioleciiles. f airdischarge .produces A range since vthe photon Aabs air is ra small,Under such conditions,"photo-electrcns: may be produced yat `thY eatsuch photo-electrons anode may pro'du'ce addi 1o electrons produeedfi-nt bytheanode willnA Y 4 p n mary lions created by @been v ticle whoseimpingement-is Sought tope recorded'.- Since isnch' vproportic'mality*is requisitel to proper operation *of the proportional counter; incointers. tlreproportional reeion isflirnited to the regionlying abovethe popoftinalf threshold 'and below `the pointI iwhre'ipliotons play a:signifi l-cant role.V fExperimentally, at' has "been-n found that thismnltiplication for l'gas :amplification in the' cdiintrLcontaiv-ing airatmospheric 1pm-:isa sure extends up to lappr-c atel'y 5103, whereashigher* orders of multiplication .of gas ampli: "be Thev airplacritically `del pendent upo counters vinthe and, therefere, air Vcoudependent foiie potential.

esaminano-n factor or 'i dependan aA assente field distribution, may beconsidered unimportant.

Air counters possess relatively high resolution, i. e., better than "5seconds.

In air counters heretofore employed, it has been thought that since theionization medium is air at atmospheric pressure no window should berequired, and a metallic screen has been used as all or a portionof thecathode. But, it has been found thatsuch designs have the disadvantageof the entry of clust into the high field regions and consequentvspurious counting. It is thus imperative that there be introduced abarrier for dust which is nevertheless adapted to transmit alphaparticles without substantial absorption. The best material for the dustbarrier window has been found to be sheets of nylon nlm approximately0.2 mil thick. The window of thin Amaterial suchas nylon has been foundto require a conducting coating of, .for example, graphite in order tovprevent the collection of electrostatic charge and consequent spuriouscounting. Of course, a compromise must be made between maximizingftheconductivity of the coating and minimizing the particle energyabsorption of the coating. It has been found that a satisfactory coatingmay have a resistance from the center of the window 48 to the frame 46of the order of 1,000 to- 10,000 ohms, the measurement being made bymeasuring the resistance between the framed and a coin such as a dimeplaced. at the'center of the window 48. The coating shouldhof course, beas uniform as possible, and is preferably of evaporated colloidalgraphite. .l

In operating a proportional counter filled with air at atmosphericpressure and being sufiiciently open to the atmosphere so that pressureinside Y and outside the counter is substantially the same, itis obviousthat the characteristics of the counter, which are dependent upon thepressure of the gas, will vary from day to day in accordance with thevariations in ambient atmospheric pressure. Experimentally, it has beenfound that with counters using 1 mil diameter centerwires the shift inthreshold potential in response to an atmospheric pressure change of 1cm. Hg (the maximum daily fluctuation of atmospheric pressure in mostlocations). is approximately 15 to 2G Volts, the normal operatingpotential being in the neighborhood of 21200 volts. This shift islinear' over a range of at least 20 cm. Hg in the vicinity of sea levelat atmospheric pressure. It has been found 'that in any given locationthe shift in operating'characteristics due to atmospheric pressurechanges can be virtually disregarded when the counter is incorporatedinV instruments Afor alpha' particle and proton.

surveying and detection, which do not require extreme accuracy. At roomtemperatures, it has been found that there is vapproximately a 1 percent change in the counting rate of the counter with a 1 F. change intemperature.

A thin window of, for example, nylonhas afairly high permeability toVwater vapor. The

water vvapor content'of air changes from'time ing'in a counter with ahigh background, ex-f'v cept that the counts are somewhat periodicrather than random in occurrence. Counters with fairly clean insulatorsmay show the effect above per cent relative humidity. It has been foundthat the best performance in this regard is obtained when the insulatorsare molded with proper precautions as to cleanliness rather than beingmachined. It is, of course, desirable` that the surface leakage paths onall insulators be tween the anodes and cathodes be maximized in length.In the illustration of the drawing, the gaps 22 and the illustratedtapering of the insulating posts 28 serve this purpose.

The counter illustrated in the drawing is of the flat type which has themaximum effective area in making contamination surveys on fiat surfaces.A general criterion for the proper design of the counter is that thedistance between the center-wires be approximately twice the distancebetween the parallel plates which constitute the cathode. It has beenfound that such dimensions produce a satisfactory field distribution insuch a counter without vrequiring the use of a large number ofcenter-wires.

The counter illustrated in the drawing is only one embodiment of thepresent invention. Persons skilled in the art will readily devise otherproportional counters embodying the teachings of the invention and willreadily adapt some of the features above disclosed to devices otherVthan proportional counters.

What is claimedis:

1 In an electron discharge device, in combination: an electrodecomprising aconductingwire of uniform diameter having two ends, and endportions having transverse dimensions greater than the diameter of saidwire, the respective,

wire extending through the apertures and the end portions abuttingagainst the outer surfaceY of the respective supporting members, so thatthe electrode is firmly supported but readily replaceable.

2. In an electron discharge device, in combination, an'electrodecomprising a wire having a central portion of uniform diameter and twoend portions of larger diameter than said central portion, two spacedsupport means having apertures therein larger than said central portionbut smaller than said end portion, said apertures being open at theperiphery thereof to permit peripheral introduction of the wire into theapertures, said wire extending through said apertures, and means forspringily mounting at least one of said support means so as to exerttension on said wire. 3. A proportional counter comprising, incombination, a plurality of electrodes, an ionizing medium consisting ofair at atmospheric pressure, and an enclosure surrounding saidelectrodes and said ionizing medium, said enclosure being impermeable todust of the ambient air but having at least a portion permeable to alphaparticles.

4. A proportional counter comprising in combination, a plurality ofelectrodes, an ionizing medium consisting of air at atmosphericpressure, an enclosure surrounding said electrodes and said ionizingmedium, and a window in said enclosure comprising a sheet of nylon lessthan 0.5 mil in thickness, said window being impermeable to dust of theambient air.

5. A proportional counter comprising, in combination, a conductingcenter-wire electrode, a conducting wall electrode impermeable to dustand an ionizing medium between said electrodes, said ionizing mediumconsisting of air at atmospheric pressure and said conducting Wallelectrode consisting at least in part of a thin membrane ofnon-conducting material having on at least one side thereof a thincoating of a conducting material, said membrane and coating beingpermeable to alpha particles but impermeable to dust.

6. A proportional counter in accordance with claim wherein said membraneconsists of sheet nylon less than 0.5 mil thick and said coatingcomprises evaporated colloidal graphite.

7. The apparatus of claim 1 wherein at least one of the supportingmembers is springily mounted to exert tension on the Wire.

8. The apparatus of claim 1 wherein at least one of the supportingmembers comprises a helical spring having at the end thereof asubstantially circular hook constituting said peripherally openaperture.

9. The apparatus of claim 8 wherein the end portion of the electrode issubstantially in the form of a sphere.

10. A proportional counter comprising in combination, a conductingcenter wire electrode, a conducting wall electrode impermeable to dust,and an ionizing medium between said electrodes, said ionizing mediumconsisting of a gas at substantially atmospheric pressure, and saidconducting Wall electrode consisting at least in part of a sheet ofnylon less than 0.5 mil thick having on at least one side thereof a thincoating of conducting material, said nylon sheet and coating beingpermeable to alpha particles but impermeable to dust.

11. A proportional counter comprising in combination, a conductingcenter wire electrode, a conducting Wall electrode impermeable to dust,and an ionizing medium between said electrodes, said ionizing mediumconsisting of a g-as at substantially atmospheric pressure and saidconducting electrode consisting at least in part of a thin membrane ofnon-conducting material having on at least one side thereof a thincoating of colloidal graphite, said membrane and coating being permeableto alpha particles but impermeable to dust.

12. A counter having a plurality of electrodes, an ionizing medium, anenvelope surrounding at least one of the electrodes and the ionizingmedium, and a window in the envelope permeable to radiation,characterized by the improvement wherein the Window consists of a sheetof nylon less than 0.5 mil thick coated with colloidal graphite.

13. A proportional counter comprising, in combination, a plurality ofelectrodes, an enclosure surrounding said electrodes, a window in saidenclosure, said window consisting of a thin membrane permeable to alphaparticles and impermeable to clust, an ionizable gas within theenclosure, and means for maintaining the gas at a pressure approximatelythe same as the pressure exterior of the enclosure.

JOHN A. SIMPSON, JR.

REFERENCES CITED The following references are of record. in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,562,403 Wilson Nov. 17, 19251,859,678 Nachumsohn May 24, 1932 2,130,510 Samuel Sept. 20, 19382,368,486 Mullane Jan. 30, 1945 2,397,075 Hare et al. Mar. 19, 1946FOREIGN PATENTS Number Country Date 225,293 Great Britain Dec. 1, 1924THER, REFERENCES Korn-Electron and Nuclear Counters, D. Van NostrandCo., Inc., Apr. 1946, pp. -77, 80-83 and 122.

McCusker: Journal of Scientic Instruments, Vol. 2l, July 1944, pp.12S-130.

Kori Electron and Nuclear Counters, 2D. Van Nostrand Co., Apr. 1946, pp.128 and 129.

Geiger and Muller: Article in Physikalische Zeitschrift, vol. 30, Aug.15, 1929, pp. 489-493.

Copp and Greenberg: Review o Scientic Instruments, July 1943.

Brown et al.: Review of Scientic Instruments, vol. 16, No. 5, May 1945,pp. -129.

Dunning et al.: Review of Scientific Instruments, vol. 6, Aug. 1935, pp.243-246.

Brubaker et al.: Review of Scientic Instruments, vol. 8, July 1937, pp.254258.

